The oncogenic properties of a replication-defective murine retrovirus construct, ME26, which expresses the gag-myb-ets fusion oncogene of the avian acute leukemia virus, E26, has been analyzed. Infection with this virus abrogates the serum dependence of NIH3T3 fibroblasts for growth in culture, and induces leukemia in newborn mice when introduced as an amphotropic pseudotype. In order to determine the functional role of different domains of the ME26 oncogene, mutant constructs have been generated and their biological activity analyzed. It has been demonstrated that constructs containing murine c-myb can still induce the abrogation of serum dependence in 3T3 fibroblasts and that a mutation in c-myb known to render E26 virus temperature-sensitive for transformation also appears to induce a temperature-dependent, serum-dependent growth in NIH3T3 fibroblasts. However, fusion of the human estrogen receptor binding domain to the C- terminus of the p135 coding region did not affect the ability of the resulting construct to induce proliferation of 3T3 cells in the absence of inducing hormone. These results are consistent with earlier observations that C-terminal v-myb and v-ets sequences are required for efficient alteration of growth behavior, and that alterations outside this region have little effect on the low-serum growth phenotype. Attempts have also been made to use subtractive cDNA libraries prepared from normal 3T3 RNA from 3T3 cells expressing the ME26-induced capacity to grow in low serum in order to identify the genetic targets of ME26 in these cells. Several clones, whose expression appears higher in ME26- infected cells, have been detected and will be characterized further.